As is known, a tire intended to be fitted to a heavy vehicle comprises a carcase reinforcement radially surmounted on the outside by a crown reinforcement itself surmounted by a tread the radially outermost surface of which constitutes the running surface of the tire and is intended to come into contact with the roadway when this tire is being driven on. In order to meet grip and water-clearing requirements when driving on a wet roadway, it is known practice for the tread to be provided with a plurality of essentially longitudinally (or circumferentially) directed grooves and transversely directed grooves (that is to say grooves that make a mean angle other than zero with the longitudinal direction); these grooves delimit raised elements (ribs, blocks) that constitute the tread pattern and that have a radially outermost upper face that forms a part of the running surface of the tread and lateral faces that bound the said grooves.
For certain types of heavy vehicle tire, particularly those intended for use on a roadway covered in standing water or under winter conditions, it is necessary to obtain appropriate tire performance in terms of grip and in terms of traction, while at the same time maintaining a tire life that the user will find satisfactory. To increase the grip of a tire, it is known practice to make a plurality of incisions in a plurality of tread pattern elements. What is meant here by the term incision is a kind of a cut of a width of 1 mm or less, this incision being bounded by opposing walls of rubbery material. The higher the number of incisions, the greater the number of edges formed on the tread and the greater the improvement in grip performance. What is meant by edge is a line of intersection between one of the walls delimiting an incision and the running surface of the tread.
Furthermore, double-wave incisions, that is to say incisions that have undulations in the direction of the line of the incision along the running surface and in the direction of the height of the incision are known, these allowing the rubber tread blocks to maintain a degree of rigidity and giving control over tire wear.
In order to preserve the grip performance of the tread for as long as possible as the tread wears down, it is absolutely essential to provide incisions the depth of which are equal or very similar to the depth of the longitudinal grooves (it being the latter depth that defines the working thickness of the tread in terms of the amount of rubber to be worn away).
A vehicle fitted with tires loads its tires both in the longitudinal direction of the tread (that is to say in a direction tangential to the circumferential direction of the tire) and in the transverse direction (that is to say in a direction parallel to the axis of rotation of the tire). The raised elements are loaded by forces applied by the ground to the running surface in the longitudinal direction when the tire is subjected to traction forces in order to cause the vehicle to advance or braking forces in order to reduce the speed of the said vehicle or stop it completely. These forces load the edges which are directed mainly in a transverse direction (that is to say in a direction that make an angle of less than 45 degrees with the axial direction).
The same raised elements are also loaded by forces applied by the ground to the tire in a transverse direction under cornering. These transverse forces load the edges directed mainly in the longitudinal direction (that is to say that make an angle of at most 45 degrees with the longitudinal direction).
As has already been stated, it is also absolutely essential to “set up” the tread in such a way that tread wear is as even as possible (that is to say that wear affects the entire running surface uniformly without creating patches of more pronounced wear) and for this wear to occur as slowly as possible. Thus, it is possible to give the tread a working life that is satisfactory to the user. Increasing the working life offers the user the advantage of deferring the retreading operation that consists in applying a new tread to the tire after suitable preparation.